Throughout this application, various references are referred to and disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
It is to be noted that the transitional term “comprising”, which is synonymous with “including”, “containing” or “characterized by”, is inclusive or open-ended and does not exclude additional, un-recited elements or method steps.
Graphene has various unique properties, including superior mechanical strength, low density and high heat conductivity. Many potential applications of graphene are based on its unique mechanical and electrical properties. For example, graphene oxide is water soluble with low electrical conductivity due to its large number of hydrophilic groups, such as hydroxyl, epoxide and carbonyl groups. Reduced graphene oxide, on the other hand, has good conductivity but poor solubility in water because most of the hydrophilic groups are removed during the reduction process and graphene oxide is converted to graphene with a rich π-conjugation system. Moreover, reduced graphene oxide is not compatible with other materials, such as polymers. This clearly limits its widespread use.
Several techniques have been developed to modify the surface properties of reduced graphene oxide in order to enhance its compatibility with other materials and to increase its solubility in aqueous and organic solvents. Potential techniques include (1) physically absorbing functional molecules onto the surface of graphene sheets, and (2) covalently linking functional groups onto the surface of graphene. These techniques have achieved little success in terms of broadening the use of reduced graphene oxides. For example, a dispersion of reduced graphene in aqueous solvents with other polymers has been obtained by physically absorbing aqueous soluble groups on the surface of reduced graphene oxide sheets. However, the presence of these physically absorbed compounds is not desirable for many of its potential uses. As a result, dispersions of reduced graphene oxide in aqueous solutions have remained largely unexplored. Also, due to the single layered structure with a π-conjugation system, it is difficult to disperse reduced graphene oxide in various organic solvents because the atoms in this π-conjugation system only have weak interactions with solvent molecules.
Scientists have been trying to modify graphene to influence these properties through chemical functionalization. For example, a number of methods have been developed for generating graphene and chemically modified graphene from graphite and graphene derivatives, each with its own disadvantages. (Park et al., 2009). There is a need to create new graphene derivatives having good solubility in aqueous solution and possessing other exceptional properties for various industrial applications.